1. Field of the Invention
The present invention generally relates to processing signals received with a directional antenna, and more particularly, to identifying the bearing of a signal source based on signals received with a directional antenna.
2. Description of the Related Art
As technology in air transportation has evolved, the demands on the members of the flight deck have become increasingly severe. To avoid flight path conflicts, the flight deck crew monitors considerable aircraft status information for multiple surrounding aircraft at a time when air traffic is dramatically increasing. Higher aircraft speeds magnify the burden by reducing the time in which the flight deck crew can respond to threatening situations.
To assist the flight deck crew and enhance safety, several systems have been and are being developed. Many aircraft carry transponders (e.g., mode S, mode C, mode A) by which one aircraft can communicate to a second aircraft both its identity and various flight parameters. Typically, a monitoring aircraft transmits a signal in a predetermined format which, upon receipt by an intruding aircraft, causes the intruding aircraft to respond with a transmission which includes information in a predetermined format. Systems generally referred to as traffic alert and collision avoidance systems (TCAS) process information received from intruder aircraft along with the status parameters of the receiving aircraft to identify potential collision situations. A TCAS also typically provides the flight deck crew with advisory information suggesting an action to avoid the collision situation.
A TCAS typically includes a directional antenna. The TCAS uses the directional antenna to determine the bearing of an intruder aircraft relative to the TCAS equipped monitoring aircraft. When receiving signals from the intruder aircraft, TCAS processes the signals to calculate an estimated bearing for the intruder aircraft, and this information is displayed to the flight deck crew to assist them in obtaining visual contact with the intruder aircraft.
One approach used by TCAS systems is to estimate an intruder aircraft's bearing by comparing magnitudes of signals received by the components of the directional antenna. FIG. 1 illustrates the radiation pattern of signals received by a typical directional antenna having four antenna elements measured on a test four foot diameter flat ground plane. This radiation pattern simulates the performance of the antenna on a large transport aircraft, such as an aircraft having a fuselage curvature radius greater than 80 inches. The performance of the antenna beams in each of the four quadrants representing port, starboard, fore and aft, is virtually identical. To estimate the bearing of an intruder aircraft, conventional TCAS uses a model based on the radiation pattern measured on the test ground plane, such as the radiation pattern illustrated in FIG. 1. An exemplary model used by a conventional TCAS signal processing scheme is illustrated in FIG. 2. When an intruder aircraft is detected, the bearing of the intruder aircraft is calculated by determining which beam (from the four beams representing each of the four quadrants of the polar coordinate system) has the largest amplitude and which beam has the second largest amplitude and then taking the difference: between the two. Based on this difference, a bearing estimate can be generated using a conventional TCAS model, such as the model illustrated in FIG. 2.
Various factors, however, may degrade the accuracy of the bearing estimate. For example, monitoring aircraft having small fuselages may detect transponder signals differently than larger aircraft, which may degrade the bearing estimate accuracy. Consequently, the model illustrated in FIG. 2 is not as accurate with aircraft that have small fuselages, such as those aircraft with a radius of fuselage curvature smaller than 64 inches. The smaller fuselage causes a distortion such that the beam peak in the port and starboard directions occurs at a lower elevation angle than the beams of the model illustrated in FIG. 2. This displacement may degrade the accuracy of the bearing estimate. The degradation tends to be more pronounced in aircraft with smaller fuselages than those with a larger fuselage. The amount of the error is also dependent on the elevation angle of the intruder aircraft.